HST/WFC3 Hα Direct-imaging Detection of a Pointlike Source in the Disk Cavity of AB Aur

Two of the most fundamental goals of planet formation studies are to understand how and when planets gain mass. The planetary mass assembly process can be directly constrained by characterizing actively accreting protoplanets. So far, the discoveries of accreting protoplanets are limited to PDS 70 b and c, two gas giants directly imaged within the same planetary system (Keppler et al. 2018; Haffert et al. 2019). A plethora of follow-up studies on this system confirmed the presence of circumplanetary disks (Isella et al. 2019; Benisty et al. 2021), estimated the mass accretion rates (e.g., Wagner et al. 2018; Haffert et al. 2019; Hashimoto et al. 2020; Zhou et al. 2021), investigated the planet–disk interactions (e.g., Bae et al. 2019), and constrained the planetary orbits (Wang et al. 2021). Despite this steady progress, these studies are confined to one protoplanetary system and only represent a single outcome of the planet formation process observed at an instant in time.

To expand the sample of accreting planets, we launched the Hubble Accreting Luminous Protoplanets in H-Alpha (HALPHA) Survey (Program ID: 16651, ¹² PI: Zhou) to search for accreting planets in 10 transition disk systems. We are exploiting the recently demonstrated optical and ultraviolet (UV) high-contrast imaging capability of Wide Field Camera 3 (WFC3) UVIS (Zhou et al. 2021; Sanghi et al. 2022) and using the narrowband F656N (Hα) filter to look for Hα-emitting planets. Accretion onto planets produces strong hydrogen line emission, reduces planet–star contrasts, and can in principle improve search efficiency. Any detections result in Hα luminosities (L_Hα ) and accretion rate estimates of candidate planets.

The strategy of observing transition disk systems is motivated by models showing that the disk gaps and cavities are consistent with being sculpted by multiple giant planets (Dodson-Robinson & Salyk 2011; Close 2020). These models are broadly supported by the discoveries and characteristics of PDS 70 b and c (Bae et al. 2019). We identified 10 systems that harbor giant cavities demonstrated in Atacama Large Millimeter/submillimeter Array (ALMA) dust continuum images (Francis & van der Marel 2020). The host stars are generally faint or located in the northern hemisphere and, consequently, are not easily accessible by ground-based visible-light adaptive -optics (AO) systems.

The first target of the survey is AB Aurigae (AB Aur), a Herbig Ae/Be star (DeWarf et al. 2003) with a spectral type of A0, a mass of 2.4 ± 0.2 M_⊙, and an isochronal age of ${6.0}_{-1.0}^{+2.5}$ Myr. ¹³ It is actively accreting at a rate of $\dot{M}\,\approx {10}^{-7}\,{M}_{\odot }{\mathrm{yr}}^{-1}$ (Garcia Lopez et al. 2006) and exhibits significant variability (>10%) in both broadband photometry and the Hα line (Harrington & Kuhn 2007; Cody et al. 2013). AB Aur hosts a transition disk that includes a highly structured inner disk with multiple spirals (e.g., Boccaletti et al. 2020; Jorquera et al. 2022) and a cavity with an outer edge at 156 au (Francis & van der Marel 2020). Several spiral structures have been identified in the inner disk and their presence has been attributed to ongoing planet formation (Fukagawa et al. 2004; Oppenheimer et al. 2008; Boccaletti et al. 2020). Specifically, Tang et al. (2017) identified two spirals in high-resolution ALMA ¹²CO J = 2 − 1 emission observations and suggested that the spiral morphology could be explained by tidal disturbance caused by a companion located at 60–80 au (04 to 06 angular separation) and a ∼180° position angle (P.A.). Recently, Currie et al. (2022) presented evidence for an embedded protoplanet, referred to as AB Aur b, in visible and infrared images near the location predicted by Tang et al. (2017). In this Letter, we report results from two epochs of Hubble Space Telescope (HST)/WFC3 F656N (Hα) high-contrast imaging observations of AB Aur.

2.1. HST Observations

2.2. High-resolution Spectroscopy of AB Aur

Our data reduction starts with the flc files downloaded from the HST archive. The nominal bias, dark, flat-field calibrations, as well as charge transfer efficiency correction are performed by the CalWFC3 pipeline. We first identify cosmic-ray (CR) affected pixels using the data quality flag (DQ = 4096) and replace these pixels with two-dimensional linear interpolations of the respective neighboring pixels. Every set of four-point-dithered and CR-corrected frames are processed by a Fourier-interlacing pipeline (Lauer 1999; Zhou et al. 2021) to construct one 2 × finer sampled image. This pipeline is optimized for point-source PSFs and is distinct from the drizzling method that is part of the regular data reduction performed at STScI.

We then independently perform a complete set of primary subtraction procedures on the natively sampled (pixel scale = 40 mas) and reconstructed (pixel scale = 20 mas) image sets. First, the image cube is registered by the primary-star centroids determined by the photutils packages centroid_2dg function (Robitaille et al. 2013). Then, the images are split into two annuli that have inner/outer radii of 03/07 and 07/25, respectively. We subtract the host star PSF in each annulus using the Karhunen–Loève Image Projection (KLIP; Soummer et al. 2012) algorithm with a custom pipeline. For each image, all frames obtained at a different roll angle are used as references and the number of KLIP components is equal to the number of reference frames. We have also experimented with limiting the KLIP components to 5, 10, and 15 and find that the variation has negligible effect on the subtraction results. After primary subtraction, we rotate all frames to align their y-axes to the true north and coadd the aligned frames to form the final image. The coadding step is also performed for the two epochs individually. In total, we create six primary-subtracted images for two pixel scales each with three sets of coadds (the combined set: Figure 1; individual epochs: Figure 2).

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We convert the primary-subtracted images into signal-to-noise ratio (S/N) maps for point-source detections. The S/N is defined as the ratio between the flux integrated in a 1 FWHM radius aperture and the standard deviation of fluxes along the remaining azimuthal region sampled by nonoverlapping apertures. Biases induced by small number statistics were corrected based on Mawet et al. (2014). The S/N maps are shown in Figures 1 and 2 under the respective primary-subtracted images. Substantial residuals appear near the diffraction spikes in the reconstructed images, which are related to the telescope pointing errors.

A pointlike source emerges in every PSF-subtracted image at an identical location ∼600 mas south of the primary star. We apply the KLIP forward-modeling method (Pueyo 2016) on the natively sampled images to precisely determine the astrometry and photometry of this source and correct measurement biases introduced by primary subtraction. Synthetic WFC3 UVIS2 F656N PSFs are generated with the TinyTim package (Krist et al. 2011) as the point-source model. We inject a negative-flux PSF to the original frames and optimize the P.A., separation, and flux of the injection so that the residual sum of squares in a 3 pixel radius circular aperture minimizes.

To estimate the astrometric and photometric uncertainties, as well as the KLIP throughput, we conduct injection-and-recovery tests on the natively sampled images. Positive-flux TinyTim PSFs are injected at P.A.s of 0°, ±45°, ±90°, and ±135° spanning eight equal-interval separations between 062 and 426 (Figure 3). These PSFs have the same flux as the pointlike source. We subtract the primary PSF and then measure the positions and fluxes of the injected sources. At a specified separation, the average ratio between the injected and recovered fluxes is adopted as the throughput, and the standard deviations of the recovered fluxes, P.A.s, and separations are adopted to be the photometric and astrometric uncertainties.

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Figure 3 includes the 5σ contrast curve that characterizes our detection sensitivity at angular separations spanning 04 to 45. In deriving the contrast curve, we calculate the mean and standard deviation of the fluxes in nonoverlapping 1 FWHM radius apertures centered at a given separation. At 600 mas, the resolution element containing the detected point source is excluded. We follow the S/N definition in Mawet et al. (2014) to convert the mean and standard deviation into a flux corresponding to S/N = 5 and normalize this value by the flux of AB Aur. Finally, we divide the contrast by the KLIP throughput determined by positive PSF injections (Figure 3) to correct for flux loss in primary subtraction.

We detect a pointlike source near AB Aur in the primary-subtracted image (Figure 4). In the natively sampled frames, the detection S/Ns are 5.4, 3.8, and 6.4 in Epoch 1, Epoch 2, and the combined image, respectively; in the reconstructed images, the S/Ns are 5.6, 4.4, and 7.8. The consistent results in natively sampled and reconstructed images, as well as in individual epochs, further strengthen the confidence in the detection. The source is located at a separation of 600 ± 22 mas (93 ± 3 au in projected physical distance) away from AB Aur with a P.A. of 1825 ± 14. This position is consistent with the protoplanet candidate AB Aur b reported in Currie et al. (2022). Photometry on the combined image yields 64 ± 16 e⁻ s⁻¹, corresponding to a band-averaged flux density of F_ν = 1.5 ± 0.4 mJy or F_λ = 1.0 ± 0.3 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹. Fitting a 2D Gaussian to the source yields an FWHM of 150 mas in the tangential direction and 127 mas in the radial direction, which are 2.1 and 1.8 times the FWHM of an F656N PSF. The FWHMs of recovered artificial companions are between 85 to 120 mas. The larger size of the detected source suggests it may trace slightly extended emission.

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The Hα emission from the companion is consistent between the two epochs separated by 50 days, but our constraint on variability is weak. Photometric results of the individual epochs are 55 ± 18 e⁻ s⁻¹ and 76 ± 30 e⁻ s⁻¹. The corresponding flux densities are F_ν,1 = 1.3 ± 0.4 mJy and F_ν,2 = 1.8 ± 0.7 mJy or F_λ,1 = 0.9 ± 0.3 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹ and F_λ,2 = 1.2 ± 0.5 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹. Based on the joint flux uncertainty between the two epochs, we reject variability with amplitude greater than 50%. Using the same HST images, we precisely determine the Hα variability of the star AB Aur to be 13.1 ± 0.2%, consistent with the known variability of the host star (Harrington & Kuhn 2007; Cody et al. 2013).

Four spiral-like scattered-light features are detected at S/N values between 2 and 3. In Figure 1, we mark these structures as S1 to S4. These spiral features have all been previously identified in high-contrast imaging observations of AB Aur (e.g., Fukagawa et al. 2004; Boccaletti et al. 2020). Notably, the leading knots of S1 and S2 point toward the position of the pointlike source.

We do not detect any sources at 275 or 372 reported in Currie et al. (2022). The 5σ upper limits in our combined WFC3 frames are 0.098 mJy and 0.091 mJy, respectively, and are 4.8 and 22 times the flux measured by HST/STIS (Currie et al. 2022). Therefore, our observations do not have sufficient sensitivity to determine the nature of the two sources.

Rings and spirals in transition disks may introduce ambiguous protoplanet signals in high-contrast imaging observations. Emission from these structures may appear clumpy and even pointlike after PSF subtraction (Follette et al. 2017; Rameau et al. 2017). For example, the protoplanet candidates LkCa 15 bcd (Kraus & Ireland 2012; Sallum et al. 2015) and HD 100546 bc (Currie et al. 2015) were later found to be consistent with disk features (LKCa 15: Currie et al. 2019; HD 100546: Follette et al. 2017; Rameau et al. 2017). Because Hα emission from an accreting planet distinguishes the planet from the disk in their spectral energy distributions (SEDs), Hα images can provide an independent way of identifying protoplanets. For PDS 70 b and c, their Hα flux densities are over 100 times that of the estimated optical continuum, which help confirm these objects as accreting protoplanets. In contrast, in the HD 100546 case, nondetections of Hα-emitting point sources call into question the candidates as accreting protoplanets (Follette et al. 2017; Rameau et al. 2017). For AB Aur b, Currie et al. (2022) presented several lines of evidence to support the protoplanet identification: orbital motion has been detected, and the source's spectrum and polarimetric intensities differ from the rest of the disk. However, the companion resides in a region of extended scattered light and photospheric features appear to be absent in the SED (Currie et al. 2022). Below, we discuss whether the Hα detection adds support to the protoplanet interpretation.

The Hα emission at the position of AB Aur b has been detected by Subaru/VAMPIRES (Currie et al. 2022) and HST/WFC3 (this work). However, it is unclear whether the VAMPIRES detection is physically associated with AB Aur b due to uncertainties in the instrument's astrometric calibration (Currie et al. 2022). The astrometry of the WFC3 detection is well calibrated and consistent with measurements in other bands. Therefore, we only adopt the WFC3 measurement for this discussion. The WFC3 band-averaged Hα flux density of f_ν = 1.5 ± 0.4 mJy is higher than the optical continuum measured by HST/STIS (f_ν = 0.47 ± 0.05 mJy; λ = 0.57 μm, Δλ = 0.43 μm) and Subaru/VAMPIRES (f_ν = 1.13 ± 0.37 mJy; λ = 0.647 μm), but the difference is moderate to marginal, respectively. The line-to-continuum flux ratio (defined as the band-averaged F656N flux divided by the continuum flux) is 3.2 ± 0.9 or 1.3 ± 0.6, depending on whether the STIS or the VAMPIRES flux is adopted for the continuum, much lower than the flux ratios >100 estimated for the PDS 70 planets, as well as several accreting planetary mass companions (Zhou et al. 2014; Eriksson et al. 2020; Stolker et al. 2021). AB Aur b's low line-to-continuum flux ratio is inconsistent with the accretion shock models that explains the observations of PDS 70 b and c (e.g., Aoyama et al. 2018), which produce pronounced Hα lines. As a result, the Hα detection does not strengthen the interpretation that AB Aur b is an accreting protoplanet.

If the observed Hα emission is indeed from an accretion shock, the shock also produces strong optical continuum emission that reduces the line-to-continuum flux ratio. Follow-up characterization of this source to measure ultraviolet and optical excess emission would provide useful comparisons to model SEDs and enable a diagnosis of the accretion shock interpretation (e.g., Zhu 2015). Assuming that the observed F656N flux is entirely produced by the accretion shock of AB Aur b, we can determine the instantaneous mass accretion rate of the protoplanet based on the observed Hα luminosity. Adopting a line-of-sight extinction of A_V = 0.5 mag (the same as the star; Garcia Lopez et al. 2006) and assuming no additional extinction opacity, we find a Hα line luminosity of L_Hα = 2.2 ± 0.7 × 10⁻⁵ L_⊙. At this L_Hα , gas in the accretion flow should not lead to significant absorption (Marleau et al. 2022). The L_Hα value corresponds to a total accretion luminosity of $\mathrm{log}({L}_{\mathrm{acc}}/{L}_{\odot })=-2.8\pm 0.3$ by assuming the planetary surface shock model (Aoyama et al. 2018, 2021) or $\mathrm{log}({L}_{\mathrm{acc}}/{L}_{\odot })=-3.5\pm 0.3$ using the empirical relation for classic T Tauri stars (Alcalá et al. 2017), respectively. Adopting a planetary mass of 9M_Jup and radius of 2.7R_Jup (Currie et al. 2022), these accretion luminosities can be translated to mass accretion rates of $\mathrm{log}(\dot{M}/({M}_{\mathrm{Jup}}\,{\mathrm{yr}}^{-1}))$ between −5.8 and −6.6. These estimates roughly agree with the result $\mathrm{log}(\dot{M}/({M}_{\mathrm{Jup}}\,{\mathrm{yr}}^{-1}))=-6.0$ obtained from SED fitting (Currie et al. 2022).

Alternatively, the observed Hα flux can also be stellar light scattered by a compact disk structure or an envelope surrounding the protoplanet. In this case, the optical SED of the candidate protoplanet, which requires additional multiband photometry to precisely determine, should mimic the SED of the star. Remarkably, the line-to-continuum ratios of AB Aur (2.58) and AB Aur b (3.2 ± 0.9, adopting HST/STIS photometry as the continuum) are consistent within 1σ (Figure 5, bottom right panel). We note that this agreement might be serendipitous because AB Aur's Hα emission is variable (Harrington & Kuhn 2007) and our comparison is based on asynchronous observations (the stellar spectrum is obtained 15 months earlier than the HST images). Our HST observations do not measure continuum flux and thus cannot simultaneously constrain the stellar line-to-continuum flux ratios. Continuous and simultaneous Hα monitoring of AB Aur and AB Aur b can help test this scattering scenario. Currie et al. (2022) concluded that the scattered light could not account for the infrared emission, because AB Aur b was undetected in infrared polarized images. If the Hα flux is scattered light, the disk structure or dust envelope should predominantly scatter at short wavelengths such that scattered stellar emission is only significant in optical bands.

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Finally, we emphasize the striking difference in the SED shape between AB Aur b and the PDS 70 planets. In Figure 5, we compare the SEDs of AB Aur b and PDS 70 b and find that AB Aur b is significantly brighter and bluer. Fitting single blackbody models to the SEDs of AB Aur b and PDS 70 b (excluding the Hα points) yields effective temperatures/radii of 3800 K/1.6 R_Jup and 1200 K/2.7 R_Jup, respectively. These results correspond to over 1 dex difference in their bolometric luminosities ($\mathrm{log}({L}_{\mathrm{bol}}/{L}_{\odot })=-2.3$ for AB Aur b ¹⁴ and $\mathrm{log}({L}_{\mathrm{bol}}/{L}_{\odot })=-3.8$ for PDS 70 b). We can compare the bolometric luminosity of AB Aur b to evolutionary track predictions while safely ignoring contributions from an accreting circumplanetary disk, because the expected disk luminosity is at least one order of magnitude lower. ¹⁵ Assuming a hot-start model (e.g., Burrows et al. 1997; Mordasini et al. 2017) and that AB Aur b has a planetary mass (M < 13 M_Jup), AB Aur b has to be younger than 3 Myr to have such a high L_bol. In contrast, L_bol of the PDS 70 planets are consistent with 1 to 4 M_Jup planets at an age of ∼ 5 Myr (Wang et al. 2020; Stolker et al. 2020). Because the isochronal age of AB Aur (${6.0}_{-1.0}^{+2.5}$ Myr) is older than 3 Myr, the planetary interpretation of AB Aur b and the inferred young age might imply delayed planet formation in the AB Aur disk.

1. We observed the Herbig Ae/Be star AB Aur using HST/WFC3/UVIS in the F656N (Hα) band in two epochs separated by 50 days. After subtracting the primary-star PSF, we detected a pointlike source 06 away from the star in both epochs. In the most optimal reduction, the detection S/Ns are 5.6, 4.4, and 7.8 in Epoch 1, Epoch 2, and the combined data sets, respectively.

2. The P.A. and separation of the companion are 1825 ± 14 and 600 ± 22 mas relative to the host star. This location is consistent with the astrometry of AB Aur b, a recently reported protoplanet candidate. The candidate companions at separations of 275 and 372 are below our detection limits.

3. The band-averaged Hα flux densities are 0.9 ± 0.3, 1.2 ± 0.5, and 1.0 ± 0.3 × 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹ (F_λ ) or 1.3 ± 0.4, 1.8 ± 0.7, 1.5 ± 0.4 mJy (F_ν ) in Epoch 1, Epoch 2, and the combined images, respectively.

4. In comparison to PDS 70 b and c, the Hα-to-continuum flux ratio of AB Aur b is significantly lower, suggesting that an accretion shock that produces a strong Hα line that has flux density over 100 times the continuum is not visible from AB Aur b. Both planetary accretion and scattered stellar emission from either a compact disk structure or an envelope surrounding AB Aur b can contribute to the Hα flux, and the origin of the Hα emission remains unconstrained.

5. Assuming that the observed Hα emission is entirely powered by accretion onto the protoplanet and there is no gas or dust extinction from the disk nor from the accretion flow itself, we estimate a mass accretion rate of $\mathrm{log}(\dot{M}/{M}_{\mathrm{Jup}}{\mathrm{yr}}^{-1})=-5.8$ to −6.6, depending on the assumed L_Hα –L_acc relations. The accretion rate estimate suffers from significant systematic uncertainty due to the unknown origin of the Hα emission and unconstrained extinction.

The detection of Hα emission from the candidate protoplanet AB Aur b offers exciting opportunities to understand the mass assembly processes of giant planets forming in a highly structured disk. These interpretations are conditioned on definitively resolving the origin of the Hα emission. Disentangling the planetary accretion versus stellar scattering of Hα emission possibilities could not only prove to be crucial in understanding the origin of AB Aur b, but also for the interpretation of future discoveries of protoplanets embedded in the disks of their host stars.

We thank the referee for a constructive report that improves the rigor of the manuscript. We acknowledge excellent observing support from STScI staff, especially J. Green and A. Armstrong. We thank Thayne Currie for the CHARIS spectrum data and Gabriel-Dominique Marleau for helpful comments. Y.Z. acknowledges support from the Heising-Simons Foundation 51 Pegasi b Fellowship. B.P.B. acknowledges support from the National Science Foundation grant AST-1909209, NASA Exoplanet Research Program grant 20-XRP20_2-0119, and the Alfred P. Sloan Foundation. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. The observations and data analysis works were supported by program HST-GO-16651. Supports for Program numbers HST-GO-16651 were provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

Software: Astropy (Robitaille et al. 2013), Numpy (van der Walt et al. 2011), Scipy (Virtanen et al. 2020), Matplotlib (Hunter 2007), pyKLIP (Wang et al. 2015).